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A time zone is a region of the earth that has uniform standard time, usually referred to as the local time. By convention, time zones compute their local time as an offset from UTC (see also Greenwich Mean Time). Local time is UTC plus the current time zone offset for the considered location.

Introduction

Time zones are divided into standard and daylight saving (or summer). Daylight saving time zones (or summer time zones) include an offset (typically +1) for daylight saving time.

Standard time zones can be defined by geometrically subdividing the Earth's spheroid into 24 lunes (wedge-shaped sections), bordered by meridians each 15° of longitude apart. The local time in neighboring zones would differ by one hour. However, political boundaries, geographical practicalities, and convenience of inhabitants can result in irregularly-shaped zones. Moreover, in a few regions, half-hour or quarter-hour differences are in effect.

Before the adoption of time zones, people used local solar time. Originally this was apparent or true solar time, as shown by a sundial, and later it became mean solar time, as kept by most mechanical clocks. Mean solar time has days of equal length, but the difference between mean and apparent solar time, called the equation of time, averages to zero over a year.

The use of local solar time became increasingly awkward as railways and telecommunications improved, because clocks differed between places by an amount corresponding to the difference in their geographical longitude, which was usually not a convenient number. This problem could be solved by synchronizing the clocks in all localities, but in many places the local time would then differ markedly from the solar time to which people were accustomed. Time zones are a compromise, relaxing the complex geographic dependence while still allowing local time to approximate the mean solar time. There has been a general trend to set the boundaries of time zones west of their designated meridians in order to create a permanent daylight saving time effect. The increase in worldwide communication has further increased the need for interacting parties to communicate mutually comprehensible time references to one another.

Standard time zones

Standard time zones of the world as of June 2008

Until fairly recently, time zones were based on Greenwich Mean Time (GMT, also called UT1), the mean solar time at longitude 0° (the Prime Meridian). But as a mean solar time, GMT is defined by the rotation of the Earth, which is not constant in rate. So, the rate of atomic clocks was annually changed, or steered, to closely match GMT. In January 1972, however, atomic clock rates were fixed and predefined leap seconds replaced rate changes. This new time system is called Coordinated Universal Time (UTC). Leap seconds are inserted to keep UTC within 0.9 seconds of UT1. In this way, local times continue to correspond approximately to mean solar time, while the effects of variations in Earth's rotation rate are confined to simple step changes that can be more easily applied to obtain a uniform time scale (International Atomic Time or TAI). With the implementation of UTC, nations began to use it in the definition of their time zones instead of GMT. As of 2005, most but not all nations had altered the definition of local time in this way (though many media outlets fail to make a distinction between GMT and UTC). Further change to the basis of time zones may occur if proposals to abandon leap seconds succeed.

Notation of time (ISO 8601)

UTC

If the time is in UTC, add a 'Z' directly after the time without a space. 'Z' is the zone designator for the zero UTC offset. "09:30 UTC" is therefore represented as "09:30Z" or "0930Z". "14:45:15 UTC" would be "14:45:15Z" or "144515Z".

Time zone as offsets from UTC

Time zone are written as offset from UTC in the format ±[hh]:[mm], ±[hh][mm], or ±[hh]. So if the time being described is one hour ahead of UTC (such as the time in Berlin during the winter), the zone designator would be "+01:00", "+0100", or simply "+01". This is appended to the time in the same way that 'Z' was above. The offset from UTC changes with daylight saving time, e.g. a time offset in Chicago, would be "−06:00" for the winter (Central Standard Time) and "−05:00" for the summer (Central Daylight Time).

Abbreviations

Time zones are often represented by abbreviations such as "EST, WST, CST" but these are are not part of the international time and date standard ISO 8601 and their uses as sole designator for a time zone is not recommended.

Such designation can be ambiguous as some abbreviations may refer to different timezone in other continents for example EST WST CST can refer to Eastern, Western or Central, Summer Time or Standard Time in both North America and Australia.

Examples

These examples give the local time at various locations around the world at 12:00 UTC when daylight saving time (or summer time, etc.) is not in effect:

The time-zone adjustment for a specific location may vary because of daylight saving time. For example New Zealand, which is usually UTC+12, observes a one-hour daylight saving time adjustment during the Southern Hemisphere summer, resulting in a local time of UTC+13.

For example, what time is it in Los Angeles (UTC offset= −08) when the New York Stock Exchange opens at 09:30 (−05)?

time in Los Angeles = 09:30 − (−05:00) + (−08:00) = 06:30

In Delhi (UTC offset= +5:30), the New York Stock Exchange opens at

time in Delhi = 09:30 − (−05:00) + (+5:30) = 20:00

These calculations become more complicated near a daylight saving boundary (because the UTC offset for zone X is a function of the UTC time).

History

Greenwich Mean Time (GMT) was established in 1675 when the Royal Observatory was built, as an aid to (English) mariners to determine longitude at sea. At the time, each town's local clock in the area was calibrated to its local noon. Therefore, each clock across England had a slightly different time. The first time zone in the world was established by British railway companies on December 1, 1847—with GMT kept by portable chronometers. This quickly became known as Railway Time. About August 23, 1852, time signals were first transmitted by telegraph from the Royal Observatory, Greenwich. Even though 98% of Great Britain's public clocks were using GMT by 1855, it was not made Britain's legal time until August 2, 1880. Some old clocks from this period have two minute hands—one for the local time, one for GMT.[1] This only applied to the island of Great Britain, not to the island of Ireland.

On November 2, 1868, the then-British colony of New Zealand officially adopted a standard time to be observed throughout the colony, and was perhaps the first country to do so. It was based on the longitude 172°30′ East of Greenwich, that is 11 hours 30 minutes ahead of GMT. This standard was known as New Zealand Mean Time.

Timekeeping on the American railroads in the mid 19th century was somewhat confused. Each railroad used its own standard time, usually based on the local time of its headquarters or most important terminus, and the railroad's train schedules were published using its own time. Some major railroad junctions served by several different railroads had a separate clock for each railroad, each showing a different time; the main station in Pittsburgh, Pennsylvania, for example, kept six different times. One can imagine the confusion for travelers making a long journey that involved several changes of train.

Plaque commemorating the Railway General Time Convention of 1883

Charles F. Dowd proposed a system of one-hour standard time zones for American railroads about 1863, although he published nothing on the matter at that time and did not consult railroad officials until 1869. In 1870, he proposed four ideal time zones (having north–south borders), the first centered on Washington, D.C., but by 1872 the first was centered 75°W of Greenwich, with geographic borders (for example, sections of the Appalachian Mountains). Dowd's system was never accepted by American railroads. Instead, U.S. and Canadian railroads implemented a version proposed by William F. Allen, the editor of the Traveler's Official Railway Guide.[2] The borders of its time zones ran through railroad stations, often in major cities. For example, the border between its Eastern and Central time zones ran through Detroit, Buffalo, Pittsburgh, Atlanta, and Charleston. It was inaugurated on Sunday, November 18, 1883, also called "The Day of Two Noons", when each railroad station clock was reset as standard-time noon was reached within each time zone. The zones were named Intercolonial, Eastern, Central, Mountain, and Pacific. Within one year, 85% of all cities with populations over 10,000, about 200 cities, were using standard time.[3] A notable exception was Detroit (which is about half-way between the meridians of eastern time and central time), which kept local time until 1900, then tried Central Standard Time, local mean time, and Eastern Standard Time before a May 1915 ordinance settled on EST and was ratified by popular vote in August 1916. The confusion of times came to an end when Standard zone time was formally adopted by the U.S. Congress on March 19, 1918, in the Standard Time Act.

"The question of uniform time standards for railways of the United States has long attracted the attention of railway managers, but Mr. W. F. Allen, editor of the Traveler's Official Guide, and secretary of the time conventions, is entitled to the credit of having perfected the admirable system which was adopted by the General Time Convention of Railway Managers, held at Chicago, October 11, 1883, and ratified by the Southern Railway Time Convention, held at New York, October 17, 1883. As this is a subject of great interest to the entire country, a brief synopsis of the general principles governing the proposed plan is deemed appropriate in this report.

"Under the present system each railway is operated independently on the local time of some principal point or points on said road, but this plan was found to be highly objectionable, owing to the fact that some fifty standards, intersecting and interlacing each other, were in use throughout the country. By the plan which has been adopted this number will be reduced to four, the difference in time being one hour between each, viz, the 75th, 90th, 105th, and 120th degrees of longitude west from Greenwich. The adoption of these standards will not cause a difference of more than thirty minutes from the local time at any point which is now used as a standard.

"The new arrangement goes into effect November 18, 1883, and all changes of time are to occur at the termini of roads, or at the ends of divisions. The seventy-fifth meridian being almost precisely the central meridian for the system of roads now using standards based upon the time of the Eastern cities, and the ninetieth meridian being equally central for roads now running by the time of Western cities, the time of these meridians has been adopted for the territory which includes 90 per cent. of the whole railway system of the country. Nearly all of the larger cities have abolished local time and adopted that of the nearest standard meridian in use by the railways."[4][5]

While the first person to propose a worldwide system of time zones was the Italian mathematician Quirico Filopanti, in his book Miranda! published in 1858, his idea was unknown outside the pages of his book until long after his death, so it did not influence the adoption of time zones during the 19th century. He proposed 24 hourly time zones, which he called "longitudinal days", the first centered on the meridian of Rome. He also proposed a universal time to be used in astronomy and telegraphy.[6][7]

Canadian Sir Sandford Fleming proposed a worldwide system of time zones in 1879. He advocated his system at several international conferences, thus widely credited with their invention. In 1876, his first proposal was for a global 24-hour clock, conceptually located at the center of the Earth and not linked to any surface meridian. In 1879 he specified that his universal day would begin at the anti-meridian of Greenwich (180th meridian), while conceding that hourly time zones might have some limited local use. He also proposed his system at the International Meridian Conference in October 1884, but it did not adopt his time zones because they were not within its purview. The conference did adopt a universal day of 24 hours beginning at Greenwich midnight, but specified that it "shall not interfere with the use of local or standard time where desirable".

Nevertheless, most major countries had adopted hourly time zones by 1929. Today, all nations use standard time zones for secular purposes, but they do not all apply the concept as originally conceived. Newfoundland, India, Iran, Afghanistan, Venezuela, Burma, the Marquesas, as well as parts of Australia use half-hour deviations from standard time, and some nations, such as Nepal, and some provinces, such as the Chatham Islands, use quarter-hour deviations. Some countries, most notably China and India, use a single time zone, even though the extent of their territory far exceeds 15° of longitude. Before 1949 China used five time zones (see Time in China).

Nautical time zones

Since the 1920s a nautical standard time system has been in operation for ships on the high seas. Nautical time zones are an ideal form of the terrestrial time zone system. Under the system, a time change of one hour is required for each change of longitude by 15°. The 15° gore that is offset from GMT or UT1 (not UTC) by twelve hours is bisected by the nautical date line into two 7.5° gores that differ from GMT by ±12 hours. A nautical date line is implied but not explicitly drawn on time zone maps. It follows the 180th meridian except where it is interrupted by territorial waters adjacent to land, forming gaps: it is a pole-to-pole dashed line.[8][9][10]

A ship within the territorial waters of any nation would use that nation's standard time, but would revert to nautical standard time upon leaving its territorial waters. The captain was permitted to change the ship's clocks at a time of the captain’s choice following the ship's entry into another time zone. The captain often chooses midnight.

Skewing of zones

Difference between sun time and clock time during daylight saving time:

0h ± 30m

1h ± 30m ahead

2h ± 30m ahead

3h ± 30m ahead

Ideal time zones, such as nautical time zones, are based on the mean solar time of a particular meridian located in the middle of that zone with boundaries located 7.5 degrees east and west of the meridian. In practice, zone boundaries are often drawn much farther to the west with often irregular boundaries, and some locations base their time on meridians located far to the east.

There is a tendency to draw time zone boundaries far to the west of their meridians. Many of these locations also use daylight saving time. As a result, in the summer, solar noon in the Spanish town of Muxia occurs on 14:37 by the clock. This area of Spain never experiences sunset before 18:00 (6pm) local time even in midwinter, despite its lying more than 40 degrees north of the equator. Near the summer solstice, Muxia has sunset times similar to those of Stockholm, which is in the same time zone and 16 degrees further north.

A more extreme example is Nome, Alaska, which is at 165°24′W longitude—just west of center of the idealized Samoa Time Zone (165°W). Nevertheless, Nome observes Alaska Time (135°W) with DST so it is slightly more than two hours ahead of the sun in winter and over three in summer.[11] Kotzebue, Alaska, also near the same meridian but north of the Arctic Circle, has an annual event on 9 August to celebrate two sunsets the same day, one shortly after midnight and the other shortly before midnight.

Also, China extends as far west as 73°34′E, but all parts of it use UTC+8 (120°E), so solar "noon" can occur as late as 15:00.

Daylight saving time

Many countries, or even parts of countries, adopt daylight saving time (also known as "Summer Time") during part of the year. This typically involves advancing clocks by an hour near the start of spring and adjusting back in autumn ("spring" forward, "fall" back). Some countries also use backward daylight saving over the winter period. Modern DST was first proposed in 1907 and was in widespread use in 1916 as a wartime measure aimed at conserving coal. Despite controversy, many countries have used it since then; details vary by location and change occasionally. Most countries around the equator do not observe daylight saving time, since the seasonal difference in sunlight is minimal.

Because the earliest and latest time zones are 26 hours apart, any given calendar date exists at some point on the globe for 50 hours. For example, April 11 begins in time zone UTC+14 at 10:00 UTC April 10, and ends in time zone UTC−12 at 12:00 UTC April 12.

There are numerous places where three or more time zones meet, for instance at the tri-country border of Finland, Norway and Russia.

There are 40 time zones. This is due to fractional hour offsets and zones with offsets larger than 12 hours near the International Date Line as well as one unofficial zone in Australia. See the list of time zones.

The largest time gap along a political border is the 3.5 hour gap along the border of China (UTC+08) and Afghanistan (UTC+04:30).

One of the most unusual time zones is the Australian Central Western Time zone (CWST), which is a small strip of Western Australia from the border of South Australia west to 125.5°E, just before Caiguna. It is 8¾ hours ahead of UTC (UTC+08:45) and covers an area of about 35,000 km2, larger than Belgium, but has a population of about 200. Although unofficial, it is universally respected in the area—without it, the time gap in standard time at 129°E (the WA/SA border) would be 1.5 hours. See Time in Australia.

Internet and computer systems

UTC is often used on the Internet for meetings (i.e. IRC chats, news, shows and so on).[12] For e-mail, the sender time zone is used to calculate the send time, but this time is recalculated by the receiver mail client, and shown according to the receiver time zone.

On websites with mainly domestic audiences local time is often used, sometimes with UTC in brackets: e.g. the international English-language version of CNN includes GMT and Hong Kong Time,[13] whilst the US version shows Eastern Time.[14] US Eastern Time and Pacific Time are also used fairly commonly on many US-based English-language websites with global readership.

On the other hand, most modern computer operating systems include information about time zones, including the capability to automatically change the local time when daylight saving starts and finishes (see the article on daylight saving time for more details on this aspect).

Operating systems

Unix

Most Unix-like systems, including Linux and Mac OS X, keep system time as UTC (Coordinated Universal Time). Rather than having a single time zone set for the whole computer, timezone offsets can vary for different processes. Standard library routines are used to calculate the local time based on the current timezone, normally supplied to processes through the TZ environment variable. This allows users in multiple timezones to use the same computer, with their respective local times displayed correctly to each user. Timezone information is most commonly stored in a timezone database known as tz database (or sometimes zoneinfo or Olson format). In fact, many systems, including anything using the GNU C Library, can make use of this database.

Microsoft Windows

Windows-based computer systems normally keep system time as local time in a particular time zone. A system database of timezone information includes the offset from UTC and rules that indicate the start and end dates for daylight saving in each zone. Application software is able to calculate the time in various zones, but there is no standard way for users from multiple zones to use a single server and have their own local times presented to them other than with Windows 2003 Terminal Server. Windows 2003 Terminal Servers allow remote computers to redirect their time zone settings to the Terminal Server so that users see the correct time for their time zone in their desktop/application sessions. Terminal Services uses the server base time on the Terminal Server and the client time zone information to calculate the time in the session. By default, this feature is disabled.

Programming languages

Java

While most application software will use the underlying operating system for timezone information, the Java Platform, from version 1.3.1, has maintained its own timezone database. This database will need to be updated whenever timezone rules change. Sun provides a updater tool for this purpose.[15]

As an alternative to the timezone information bundled with the Java Platform, programmers may choose to use the Joda-Time library.[16] This library includes its own timezone data based on the frequently-updated tz database.[17]

Javascript

There is very little in the way of timezone support for Javascript. Essentially the programmer has to extract the UTC offset by instantiating a time object, getting a GMT time from it, and differencing the two. This does not provide a solution for daylight savings variations.

PHP

The DateTime objects and related functions have been compiled into the PHP core since 5.2. This includes the ability to get and set the default script timezone, and DateTime is aware of its own timezone internally. PHP.net provides extensive documentation on this.[18] As noted there, the most current timezone database can be implemented via the PECL timezonedb.

Python

The standard module datetime stores and operates on the timezone information class tzinfo. The third party pytz module provides access to the full tz database.[19]. Negated time zone offset in seconds is stored time.timezone and time.altzone attributes.

Smalltalk

Each Smalltalk dialect comes with its own built-in classes for dates, times and timestamps, only a few of which implement the DateAndTime and Duration classes as specified by the ANSI Smalltalk Standard. VisualWorks provides a TimeZone class that supports up to 2 annually-recurring offset transitions, which are assumed to apply to all years (same behavior as Windows time zones.) Squeak provides a Timezone class that does not support any offset transitions. Dolphin Smalltalk does not support time zones at all.

For full support of the tz database (zoneinfo) in a Smalltalk application (including support for any number of annually-recurring offset transitions, and support for different intra-year offset transition rules in different years) the third-party, open-source, ANSI-Smalltalk-compliant Chronos Date/Time Library is available for use with any of the following Smalltalk dialects: VisualWorks, Squeak or Dolphin.

Databases

Some databases allow storage of a datetime type having time zone information. The SQL standard defines two standard time data types:

TIMESTAMP WITH TIME ZONE

TIMESTAMP WITHOUT TIME ZONE

However, the standard has a somewhat naive understanding of time zones. It generally assumes a time zone can be specified by a simple offset from GMT. This causes problems when trying to do arithmetic on dates which span daylight saving time transitions or which span political changes in time zone rules.

Oracle

Oracle Database is configured with a database time zone, and connecting clients are configured with session time zones. Oracle Database uses two data types to store time zone information:

TIMESTAMP WITH TIME ZONE

Stores date and time information with the offset from UTC

TIMESTAMP WITH LOCAL TIME ZONE

Stores date and time information with respect to the dbtimezone (which cannot be changed so long as there is a column in the db of this type), automatically adjusting the date and time from the stored time zone to the client's session time zone.

PostgreSQL

PostgreSQL uses the standard SQL data types but tries to impose an interpretation which avoids the problems described above.

TIMESTAMP WITH TIME ZONE

Stores date and time in UTC and converts to the client's local time zone (which could be different for each client) for display purposes.